This research is directed toward the synthesis and full electronic, structural, and reactivity characterization of low molecular weight iron-sulfur complexes which serve as close representations of the active sites of iron-sulfur proteins. The complexes (Fe4S4(SR)4)2- and (FeS(SCH2)2C6H4)2- have been prepared and characterized, and are analogs of the active sites of 2-Fe and 4- and 8-Fe proteins, respectively. In addition to continuing study of these species, the following research will be undertaken: (1) synthesis and characterization of an active site analog of rubredoxin (1-Fe) proteins; (2) synthesis and characterization of the generalized tetramer series (Fe4X4(YR)4)2-, X equals O, S, Se; Y equals O, S, Se, Te; (3) isolation and structure determination of (Fe4S4(SR)4)3-,1- thereby completing identification of all members of the synthetic electron transfer series for which there are proteins with equivalent oxidation levels; (4) reconstitution of proteins from apoproteins and preformed Fe-S clusters, extrusion of these clusters from holoproteins using aromatic thiols; (5) elucidation of the mechanism of ligand substitution reactions of (Fe4S4(SR)4)2- with thiols by stopped-flow kinetic measurements; (6) X-alpha scattered wave calculations of the electronic structure of (Fe4S4(SR)4)2-; (7) studies of the ligand substitution reactions of (Fe2S2(SR)4)2- and electronic properties of the reduced trimer dianion.